Asynchronous JavaScript and XML (AJAX) is not a technology in itself, but is a term that describes a "new" approach to using a number of existing technologies together, including: HTML or XHTML, Cascading Style Sheets, JavaScript, The Document Object Model, XML, XSLT, and the XMLHttpRequest object. When these technologies are combined in the AJAX model, web applications are able to make quick, incremental updates to the user interface without reloading the entire browser page. This makes the application faster and more responsive to user actions.

Advantages of AJAX

Bandwidth usage

By generating the HTML locally within the browser, and only bringing down JavaScript calls and the actual data, Ajax web pages can appear to load relatively quickly since the payload coming down is much smaller in size, and the rest of the layout does not have to be redrawn on each update. An example of this technique is a large result set where multiple pages of data exist. With Ajax, the HTML of the page (e.g., a table structure with related TD and TR tags) can be produced locally in the browser and not brought down with the first page of the document. In addition to "load on demand" of contents, some web-based applications load stubs of event handlers and then load the functions on the fly. This technique significantly cuts down the bandwidth consumption for web applications. On the other hand, Ajax works on the client and shares some work of server, so that reduces the server load.

Separation of data, format, style, and function

A less specific benefit of the Ajax approach is that it tends to encourage programmers to clearly separate the methods and formats used for the different aspects of information delivery via the web. Although Ajax can appear to be a jumble of languages and techniques, and programmers are free to adopt and adapt whatever works for them, they are generally propelled by the development motive itself to adopt separation among the following:

1. Raw data or content to be delivered, which is normally embedded in XML and sometimes derived from a server-side database.
2. Format or structure of the webpage, which is almost always built in HTML or XHTML and is then reflected and made available to dynamic manipulation in the DOM.
3. Style elements of the webpage: everything from fonts to picture placement are derived by reference to embedded or referenced CSS.
4. Functionality of the webpage, which is provided by a combination of:
1. Javascript on the client browser (Also called DHTML),
2. Standard HTTP and XMLHttp or client-to-server communication, and
3. Server-side scripting and/or programs using any suitable language preferred by the programmer to receive the client’s specific requests and respond appropriately.

Disadvantages of AJAX

Browser integration

The dynamically created page does not register itself with the browser history engine, so triggering the "Back" function of the users’ browser might not bring the desired result.

Developers have implemented various solutions to this problem. These solutions can involve using invisible IFRAMEs to invoke changes that populate the history used by a browser’s back button. Google Maps, for example, performs searches in an invisible IFRAME and then pulls results back into an element on the visible web page. The World Wide Web Consortium (W3C) did not include an IFRAME element in its XHTML 1.1 Recommendation; the Consortium recommends the object element instead.

Another issue is that dynamic web page updates make it difficult for a user to bookmark a particular state of the application. Solutions to this problem exist, many of which use the URL fragment identifier (the portion of a URL after the ‘#'[7][8]) to keep track of, and allow users to return to, the application in a given state. This is possible because many browsers allow JavaScript to update the fragment identifier of the URL dynamically, so that Ajax applications can maintain it as the user changes the application’s state. This solution also improves back-button support. It is not, however, a complete solution.

Response-time concerns

Network latency — or the interval between user request and server response — needs to be considered carefully during Ajax development. Without clear feedback to the user,[9] preloading of data and proper handling of the XMLHttpRequest object, users might experience delays in the interface of the web application, something which they might not expect or understand. Additionally, when an entire page is rendered there is a brief moment of re-adjustment for the eye when the content changes. The lack of this re-adjustment with smaller portions of the screen changing makes the latency more apparent. The use of visual feedback (such as throbbers) to alert the user of background activity and/or preloading of content and data are often suggested solutions to these latency issues.

Search engine optimization

Websites that use Ajax to load data which should be indexed by search engines must be careful to provide equivalent Sitemaps data at a public, linked URL that the search engine can read, as search engines do not generally execute the JavaScript code required for Ajax functionality. This problem is not specific to Ajax, as the same issue occurs with sites that provide dynamic data as a full-page refresh in response to, say, a form submit (the general problem is sometimes called the hidden, or deep web).

Reliance on JavaScript and the DOM

Ajax relies on JavaScript and the browser’s Document Object Model (DOM), which are often implemented differently by different browsers or versions of a particular browser. Because of this, sites that use JavaScript may need to be tested in multiple browsers to check for compatibility issues. It’s common to see JavaScript code written twice, one part for IE, another part for Mozilla compatibles, although this is less true with the release of IE7 and with the now-common use of JavaScript abstraction libraries like the Prototype JavaScript Framework or Jquery. Such libraries abstract browser-specific differences from the web developer.

The level of IDE support for JavaScript used to be poor, although it is changing with more wide-spread use of tools like firebug, IE Developer Toolbar and Venkman.

An issue also arises if the user has switched off JavaScript support in the browser, thus disabling the functionality built into the pages.

Web analytics

Many web analytics solutions are based on the paradigm of a new page being loaded whenever new or updated content is displayed to the user, or to track a series of steps in a process such as a check-out. Since Ajax alters this process, care must be taken to account for how to instrument a page or a portion of a page so that it can be accurately tracked. Analytics systems which allow for the tracking of events other than a simple page view, such as the click of a button or link, are the ones most likely to be able to accommodate a site which heavily utilizes Ajax.